1. Field of the Invention
The present invention relates to proteins and nucleic acids related to salt tolerance in plants.
2. Description of the Background
Soil salinity is a major abiotic stress for plant agriculture. Sodium ions in saline soils are toxic to plants due to its adverse effects on K+ nutrition, cytosolic enzyme activities, photosynthesis and metabolism (1, 2). Three mechanisms function cooperatively to prevent the accumulation of Na+ in the cytoplasm, i.e. restriction of Na+ influx, active Na+ efflux and compartmentation of Na+ in the vacuole (1). The wheat high-affinity K+ transporter HKT1 functions as a Na+-K+ cotransporter, which confers low-affinity Na+ uptake at toxic Na+ concentrations (3). Thus HKT1 could represent one of the Na+ uptake pathways in plant roots. The low-affinity cation transporter LCT1 from wheat may also mediate Na+ influx into plant cells (4). In addition, patch clamp studies have shown that non-selective cation channels play important roles in mediating Na+ entry into plants (5). The Arabidopsis thaliana AtNHX1 gene encodes a tonoplast Na+/H+ antiporter and functions in compartmentalizing Na+ into the vacuole (6). Over-expression of AtNHX1 enhances the salt tolerance of Arabidopsis plants (7).
No Na+ efflux transporter has been cloned from plants. Plants do not appear to have a Na+-ATPase at the plasma membrane (1). It is expected that proton motive force created by H+-ATPases would drive Na+ efflux from plant cells through plasma membrane Na+/H+antiporters (8). Fungal cells contain both Na+-ATPases and Na+/H+ antiporters at the plasma membrane. In the yeast Saccharomyces cerevisiae, plasma membrane Na+′-ATPases play a predominant role in Na+ efflux and salt tolerance (9). In contrast, Na+/H+ antiporters are more important for Na+ efflux and salt tolerance in the fungus Schizosaccharomyces pombe (10).
Recently, several Arabidopsis sos (for salt overly sensitive) mutants defective in salt tolerance were characterized (11, 12, 13). The sos mutants are specifically hypersensitive to high external Na+ or Li+ and also unable to grow under very low external K+ concentrations (13). Allelic tests indicated that the sos mutants define three SOS loci, i.e., SOS1, SOS2 and SOS3 (13). The SOS3 gene encodes an EF-hand type calcium-binding protein with similarities to animal neuronal calcium sensors and the yeast calcineurin B subunit (14). In yeast, calcineurin plays a central role in the regulation of Na+ and K+ transport. Mutations in calcineurin B lead to increased sensitivity of yeast cells to growth inhibition by Na+ and Li+ stresses (15). The SOS2 gene was recently cloned and shown to encode a serine/threonine type protein kinase (16). Interestingly, SOS2 physically interacts with and is activated by SOS3 (17). Therefore, SOS2 and SOS3 define a novel regulatory pathway for Na+ and K+ homeostasis and salt tolerance in plants. The SOS3/SOS2 pathway has been predicted to control the expression and/or activity of ion transporters (17). However, the identities of the transporters regulated by this pathway are not known.
Among the three SOS loci, SOS1 plays the greatest role in plant salt tolerance. Compared to sos2 and sos3 mutant plants, sos1 mutant plants are even more sensitive to Na+ and Li+ stresses (13). Double mutant analysis indicated that SOS1 functions in the same pathway as SOS2 and SOS3 (12, 13). Thus, SOS1 may be a target for regulation by the SOS3/SOS2 pathway.
Accordingly, there remains a need in the art to isolate the SOS1 gene and the protein encoded thereby.
Furthermore, because of limited water supplies and the widespread use of irrigation, the soils of many cultivated areas have become increasingly salinized. In particular, modern agricultural practices such as irrigation impart increasing salt concentrations when the available irrigation water evaporates and leaves previously dissolved salts behind. As a result, the development of salt tolerant cultivars of agronomically important crops has become important in many parts of the world. For example, in salty soil found in areas such as Southern California, Arizona, New Mexico and Texas.
Dissolved salts in the soil increase the osmotic pressure of the solution in the soil and tend to decrease the rate at which water from the soil will enter the roots. If the solution in the soil becomes too saturated with dissolved salts, the water may actually be withdrawn from the plant roots. Thus the plants slowly starve though the supply of water and dissolved nutrients may be more than ample. Also, elements such as sodium are known to be toxic to plants when they are taken up by the plants.
Salt tolerant plants can facilitate use of marginal areas for crop production, or allow a wider range of sources of irrigation water. Traditional plant breeding methods have, thus far, not yielded substantial improvements in salt tolerance and growth of crop plants. In addition, such methods require long term selection and testing before new cultivars can be identified.
Accordingly, there is a need to increase salt tolerance in plants, particularly those plants which are advantageously useful as agricultural crops.